In an electrophotographic image forming apparatus, such as a printer, a copy machine, a facsimile machine, and a multifunction machine having the functions of these machines, the amount of toner (e.g., the toner density) contained in a toner image formed on a photosensitive member, such as a photosensitive drum, directly affects the quality of a printed image. The toner density varies in accordance with various conditions, such as the environment in which the image forming apparatus is used and also the duration of use. For example, when charging characteristics of a developing agent vary in accordance with a variation in the ambient environment, it becomes difficult for toner to reliably travel from a development unit to the photosensitive member if a constant developing bias is applied to the photosensitive member.
In addition, as the number of times an image forming process has been carried out increases, the thickness of a photosensitive layer on the photosensitive member decreases as a result of abrasion caused by contact with a cleaning blade for cleaning the photosensitive member or with an intermediate transfer member. Accordingly, it becomes difficult to maintain a constant surface potential of the photosensitive member. When the surface potential gradually decreases, the toner image density increases. As a result, the image quality becomes degraded.
In particular, in a tandem color image forming apparatus having a plurality of photosensitive members corresponding to colors arranged along a moving direction of an intermediate transfer member, there is a risk that an image having colors different from the desired colors will be formed.
To prevent this, some image forming apparatuses according to the related art adjust the toner image density as described below. As illustrated in FIGS. 1A and 1B, toner image 103 of a toner-density-adjustment image (e.g., a test pattern) is formed in predetermined area 102 of, for example, intermediate transfer belt 101 onto which a toner image formed on the photosensitive member is transferred first. The density of toner image 103 of the test pattern is measured by a sensor. The toner image density is adjusted by controlling process conditions, such as a developing bias voltage, in accordance with the result of the measurement. The test pattern includes, for example, areas in which the toner density changes stepwise.
The result of the measurement of the test pattern is affected by the reflectance of the background, that is, the reflectance of the surface of the intermediate transfer member in the area where the test pattern is to be formed. In the above-described tandem color image forming apparatus, the surface of the inter mediate transfer member, such as the intermediate transfer belt, comes into contact with, for example, a cleaning member and a transfer roller that transfers the toner image onto a recording medium, such as printing paper. Therefore, the surface of the intermediate transfer member is generally stained or scratched. In addition, a toner additive or the like adheres to the surface of the intermediate transfer member. Therefore, the measured toner density (as depicted in FIG. 1B) may be inaccurate. For this reason, a surface state of intermediate transfer member 101 before the formation of test pattern 103 (as shown in FIG. 1A) is measured in advance with the sensor, and the toner density is corrected on the basis of a sensor value obtained by the sensor.
The developing agent used for forming the toner image includes toner and a carrier. The toner additive, such as titanium oxide, is added to the toner. In the developing process for forming the toner image, a developing bias and a primary transfer bias are applied to the photosensitive member. At this time, some of the toner additive may be discharged separately from the toner and adhere to the surface of the intermediate transfer member.
In such a case, the toner additive adheres also to the background on which the test pattern is formed. Therefore, the reflectance and the like of the background cannot be accurately measured by the sensor and it becomes difficult to accurately measure the toner density.